The present invention relates to a process for treating a liquid waste containing solid fine particles, and particularly to a process for separating and removing solid fine particles having sizes of 0.1-10 .mu.m, so-called cruds, suspended in the backwash water from non-filter aid type filter used in an apparatus for treating radioactive liquid waste in the primary cooling water system of an atomic power plant.
The radioactive liquid waste discharged from the primary cooling water system of an atomic power station contains insoluble cruds comprised mainly of radioactivated iron oxide fine particles, and thus it is necessary to separate and remove the cruds from the liquid waste by a radioactive liquid waste filter to reuse the liquid waste as primary cooling water.
As the radioactive liquid waste filter, a non-filter aid type filter, for example, a membrane filter, such as nuclear pore membrane filter, a centrifugal filter, such as supercentrifuge, or a porous tubular filter, has been recently used to reduce the amount of secondary waste.
The radioactive liquid waste from the primary cooling water system of an atomic power plant has been so far treated in the following manner.
A liquid waste from the primary cooling water system of an atomic power plant, which contains insoluble cruds, is collected in a liquid waste collection tank, and then filtered through a non-filter aid type filter so that the filtrate can have a crud concentration of the standard value of 0.5 ppm or less, and then the filtrate is demineralized. The dimineralized filtrate is collected into a liquid sample tank, and when the filtrate meets the recovery standard value by sampling, the filtrate is recovered into a recovered water storage tank, and then reused as the primary cooling water for the atomic power plant.
The non-filter aid type filter, which has captured the cruds in the above-mentioned treatments, is controlled to refrain passage of the liquid waste therethrough at specific intervals and is backwashed with water to separate the captured cruds and maintain its performance. The backwash water from the non-filter aid type filter is transferred into a waste resin settling tank for backwash water (comprised mainly of waste ion exchange resin powder) from the filtrate-demineralizer of the nuclear reactor coolant clean-up system in a waste treatment facility (the nuclear reactor coolant clean-up system will be hereinafter referred to as "CUW system"), where the cruds are allowed to settle. The resulting supernatant is recycled to the above-mentioned liquid waste collection tank, where the turbidity of the supernatant to be recycled is set forth to be 20 ppm or less to maintain the performance of the non-filter aid type filter and satisfy the recovery standard value. When it is difficult to suppress the turbidity of supernatant to 20 ppm or less or the radioactivity of supernatant is higher, the supernatant is led to a floor drain collection tank, and then to a floor drain concentrator, where it is concentrated together with floor drain by evaporation. The water vapor thus generated is condensed, then demineralized and recovered into a floor drain sample tank. When the condensate meets the recovery standard value by sampling, the condensate is transferred into the recovered water storage tank, or discharged to the outside of the system. On the other hand, the concentrated liquid waste by the evaporation in the concentrator is stored in a concentrated liquid waste tank for a specific period of time, and then subjected to solidification treatment, and the resulting solid is filled in a drum.
It has been found by the present inventors that the supernatant treatment of the backwash water from the non-filter aid type filter has the following problems. The backwash water from the non-filter aid type filter, which is generated at least once in a day, is to be mixed with the backwash water, comprised mainly of waste ion exchange resin powder, from a filter-demineralizer of CUW system, which is generated only once in a month, in the waste resin settling tank of CUW system. That is, due to such a large difference in frequency of generation therebetween, and thus due to less chance for mixing, thorough adsorption of the cruds by the waste ion exchange resin powder cannot be expected. Consequently, fine cruds (1 .mu.m or less) remain in the supernatant, and are recycled to the liquid waste collection tank and then to the non-filter aid type filter for retreatment. Not only is the load inevitably increased on the capacity of the non-filter, aid type filter but also the clogging speed of filter is accelerated with the recycled fine cruds, thereby resulting in an increased frequency of backwashing, and correspondingly in an increased amount of backwash water.
One of measures for solving these problems is to concentrate the supernatant from the waste resin settling tank of CUW system in a concentrator by evaporation, but this inevitably increases the number of drums filled with the resulting concentrated liquid waste in a high radioactivity level.
Another measure is to use a coagulant to improve the settling ability of cruds, but the coagulant used cannot be completely removed from the supernatant, and thus the use of a coagulant has an adverse effect upon nuclear fuels, etc. when its reuse in the reactor core is taken into account.